You can check your SMTP server's event log. Usually record remains undeleted in that table if email message has not been sent successfully but still in the queue to send. You can also review columns NUM_ATTEMPTS, LAST_ATTEMPT to understand when it was happening.
It's weird that you don't see any error events in the server's application log - I guess there should be some errors logged. LAST_ATTEMPT value might be helpful to find the required event. I might be wrong.
I am not sure but the table IN_EVENT_VIEWER could contain some error records about unsent messages as well.